When recording the type of a captured variable/parameter, use the closure field it maps to. Otherwi…

tests/language/issue10721_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import "package:expect/expect.dart";
+
+void main() {
+  Expect.equals('', useParameterInClosure(1));
+  Expect.equals(43, updateParameterInClosure(1)());
+}
+
+String useParameterInClosure(arg1, {int arg2}) {
+  if (arg1 is Map) {
+    return arg1.keys.map((key) => arg1[key]).first;
+  } else {
+    return '';
+  }
+}
+
+Function updateParameterInClosure(arg1) {
+  if (arg1 is Map) {
+    return () => arg1 = 42;
+  } else {
+    return () => arg1 = arg1 + 42;
+  }
+}